```content: explain shift operator in constants example

The literal number 1267650600228229401496703205376 is potentially
as an alternative.

The == operator has not yet been used in an example in the tour,
and so using it could potentially also confuse people.

Additionally it seems useful to hint at what << and >> mean, rather
than glossing over it entirely.

Change-Id: I89652eea138a91f3da5b7629b0edfaa96dcdbc23
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ralph Corderoy <ralph@inputplus.co.uk>
```
```diff --git a/content/basics/numeric-constants.go b/content/basics/numeric-constants.go
index 97d5155..59784b5 100644
--- a/content/basics/numeric-constants.go
+++ b/content/basics/numeric-constants.go
```
```@@ -5,8 +5,11 @@
import "fmt"

const (
-	Big   = 1 << 100  // == 1267650600228229401496703205376
-	Small = Big >> 99 // == 2
+	// Create a huge number by shifting a 1 bit left 100 places.
+	// In other words, the binary number that is 1 followed by 100 zeroes.
+	Big = 1 << 100
+	// Shift it right again 99 places, so we end up with 1<<1, or 2.
+	Small = Big >> 99
)

func needInt(x int) int { return x*10 + 1 }
```